SURFACE LIGHT SOURCE DEVICE

Abstract: A surface light source device having a light conducting member made of a transparent material, a light source disposed on the light incident edge surface of the light conducting member, a light diffusing member disposed on a front surface (light emitting surface) side of the light conducting member, a reflecting member provided on a rear surface side of the light conducting member, a prismatic sheet having a multiplicity of fine prismatic surfaces arranged on the light diffusing member; a pattern having a light diffusing characteristic is provided on the surface of the light conducting member; and the pattern is formed with the aid of half-transparent ink in which fine spherical silicon grains each having a particle diameter of 1 to 10 micrometers are mixed. According to the present invention, therefore a bright surface light source device having a uniform brightness distribution can be obtained.

Description: Field of the Invention The present invention relates to a surface light source device, more particularly a surface light source device suitable for back light systems of liquid crystal displays. Background of the Invention A conventional surface light source device has a construction, for instance, as shown in Fig. 1, where a linear light source 1 such as a discharge tube, a light conducting member 2 made of a transparent material, a light diffusing plate 3, and a light reflecting member 4 are provided. The surface 2c of the light conducting member 2 is found on the side of the light reflecting member 4; and on the surface 2c, is printed a pattern 6 in white ink. In the conventional surface light source device, a light emitted from the light source 1 is made incident upon the light conducting member 2, and then the light travels inside the light conducting member 2 being fully reflected by both the surfaces 2b and 2c, which are found on the light diffusing plate 3 side and the light reflecting member 4 side of the light conducting member, respectively. While, the light traveling inside the light conducting member is emitted from the surface 2b thereof, which is situated on the light diffusing plate side of the light conducting member. The emitted light goes through the light diffusing plate 3 and then become a diffused light to constitute the surface light source. However, in the conventional surface light source device, the light is absorbed by the white ink, etc., and thus the light being emitted from the light emitting surface 2b of the light conducting member 2 is decreased. Therefore, the light cannot be used effectively so that only a dark surface light source device is obtained. Another conventional surface light source device using a light conducting member is known which has the same construction as that of the device shown in Fig. 1, where the pattern is formed on the rear surface 2c of the light conducting member 2 (the surface provided on the light reflecting member 4 side of the light conducting member) by using transparent ink in which fine spherical silicon grains are mixed. It should be noted that the ink looks half-transparent because of the mixture of the silicon grains. Such a surface light source device is known where the pattern is printed on the surface 2c of the light conducting member 2 by using the half-transparent ink having spherical silicon grains therein. In such a surface light source device using a light conducting member, only a little light is absorbed by the ink. Therefore, the amount of the light emitted from the surface 2b of the light conducting member 2 is increased as a whole. In this surface light source device, a brightness distribution, with respect to an angle  , which is formed between the light emitted from the light emitting surface 2b of the light conducting member 2 (the light L diffused by the light diffusing plate 3) and a direction perpendicular to the light emitting surface (the direction N in Fig. 1), varies depending upon the particle diameter of the silicon grains and the amount of the grains to be mixed in the ink. However, in any case, a sufficient brightness cannot be obtained in the direction perpendicular to the surface 2b of the light conducting member 2 (  = 0 degree) or in the similar direction thereof. In liquid crystal display devices, observation is generally conducted in a direction perpendicular to the liquid crystal display plate or in a similar direction thereof. Therefore, even if the surface light source device having a light conducting member, on which the pattern is printed by using the transparent ink with said silicon grains mixed therein, is used in the liquid crystal display device as a back light system thereof, there would be a drawback in that a sufficient amount of light cannot be obtained in a direction where the observation is conducted. Further, in the above-mentioned conventional surface light source device using a light conducting member, since the light is diffused by fine light diffusing portions, which are provided on the light reflecting member side 2c of the light conducting member 2, or by the pattern formed in white or half-transparent ink, the light emitted from the light emitting surface of the light conducting member is also diffused with a great expansion. Therefore, the light emitted from the light emitting surface of the light conducting member includes not only the light directed in a direction perpendicular to the light emitting surface of the light conducting member or the similar direction thereof but also a great amount of light directed in other oblique directions. When the thus constructed surface light source device is used as a back light system of a liquid crystal display device, the light directed in the oblique directions is not useful at all for operators who observe the display in a perpendicular direction with respect to the liquid crystal display device. That is to say, the light emitted from the light conducting member can not be used effectively. Further, in the surface light source device using a light conducting member where fine light diffusing portions constitute a coarsened surface having unevenness on its cross section, which are provided on the light reflecting member side 2c of the light conducting member 2, as shown in Fig. 2, the ratio of the light emitted from the light emitting surface of the light conducting member with respect to the incident light being made incident upon the light conducting member, i.e. the usage effectivity of the light, varies depending upon the shape of the cross section of the unevenness. In this case, therefore, it becomes a matter concerning the shape of the unevenness of the coarsened surface of the fine light diffusing portion. Furthermore, in order to make the brightness greater in the surface light source device using a light conducting member, it is necessary to make the amount of light introduced from the light source 1 into the light conducting member great as considerable as possible. A device shown in Fig. 3 is known, which is arranged such that a metal foil or a film, on which silver is deposited, is formed on an inner surface of a lamp holder 20, where the light generated from the light source is introduced into the light conducting member 2 as much as possible by the reflection of the light on the metal foil or the silver film. In such a device, therefore, the efficiency of the light used is improved. On the other hand, cold cathode tubes are generally used as a light source of such surface light source devices. The reasons why are: the diameter of a cold cathode tube is generally smaller than that of a heat cathode tube, so that the size of the device as a whole can be made thin; and that since the cold cathode tubes do not generate so much heat, they can be used for a back light system of the liquid crystal display devices without giving a bad influence to the liquid crystal display devices, and thus it is easy to take care in keeping the durability of the devices high because the temperature of the tubes do not become so high. However, cold cathode tubes require a high voltage about 1 to 2 kV to be lit, depending upon the length of the tubes. The frequency thereof is generally 20 to 70 kHz. In this a manner, a high voltage should be applied in order to light the cold cathode tubes. Therefore, if a metal surface is provided on the inner surface of the lamp holder, some of the electric current flow in the cold cathode tubes leaks to the inner surface of the lamp holder, so that the brightness of the tubes is decreased, and thus, a bright surface light source cannot be obtained. Further, when some electric current leaks to the lamp holder side, electronic circuits arranged in an equipment such as a driving circuit for driving the liquid crystal display device, to which the surface light source device is mounted, are thereby affected. Further, a device is known where a sheet, mainly consisting of poly ethylene terephthalate having high reflectivity characteristics, is used as a light reflecting member. However, since the synthetic resin is used in such devices, some of the light emitted from the light source transmits through the lamp holder so that some amount of the light is lost. Therefore, a bright surface light source device cannot be obtained. On the other hand, the surface light source device disclosed in Japanese Patent Preliminarily Laid-open Application Hei4-269702 (1992) has a lamp holder in which two light reflecting surfaces are combined together. The construction of the lamp holder is shown in Fig. 4. That is to say, two light reflecting surfaces are provided inside of the lamp holder 20 and an air space 23 is formed between the light reflecting surface 21, which is provided on the light source 1 side, and the other light reflecting surface 22, which is disposed so as to surround the reflecting surface 21. The inner light reflecting surface 21 of the lamp holder is made of a metal which has a high reflectivity and the outer reflecting surface 22 is made of a white synthetic resin and arranged to work as a light diffusing reflecting surface. However, in this conventional device, if a cold cathode tube is used as a light source, some electric current leaks through the metal reflecting surface 21 disposed near the light source 1, so that the brightness of the device is decreased. In addition to this, when a metal layer is formed on a surface of a plate or a sheet made of a transparent synthetic resin as the light reflecting surface 21, it is considered that almost all of the light except for the light reflected by the metal surface does not transmit through the plate or the sheet but is absorbed by the plate or sheet which has a metal surface. Therefore, according to the device disclosed in the Publication, it is difficult to improve the using efficiency of the light by providing the two light reflecting surfaces in the lamp holder. Furthermore, in the conventional device disclosed in the Publication, the members, such as the lamp holder 20, the sheet-like light diffusing member 3, and the sheet-like light reflecting member 4, are fixed on the light emitting side and/or the opposite side (light reflecting member 4 side) of the light conducting member by means of a double-sided tape or a bonding agent, as shown by the numerical reference 30 in Figs. 5 and 6. However, on the inside surface of the double-sided tape or on the bonding agent, is provided the pattern constituted of the coarsened surfaces or a multiplicity of fine coarsened portions (fine light diffusing portions), which are formed on the rear surface 2c of the light conducting member 2 (on the surface side of the light reflecting member 4). Disclosure of the Invention The present invention has for its object to provide a surface light source device using a light conducting member, on which a pattern is printed by using transparent ink with spherical silicon grains having small diameters. According to the surface light source device of the present invention, a sufficient amount of light can be obtained in a direction perpendicular to a light emitting surface of the light conducting member. To this end, the present invention concerns a surface light source device as defined in claim 1. In the transparent ink, 20% - 80% in weight of spherical silicon grains are mixed; and each of the prisms of the prismatic sheet has an apical angle selected from a range of 80 - 100 degrees so as to correspond to the amount of spherical silicon grains mixed in the transparent ink. With such a device, the efficiency of light is improved and the amount of light directed in a direction perpendicular to the light emitting surface (surface in front of which the light diffusing member is provided) of the light conducting member becomes large. Brief Description of Drawings Fig. 1 is a cross sectional view showing a conventional surface light source device; Fig. 2 is a cross-sectional view representing another conventional surface light source device where fine light diffusing portions having unevenness on the cross section are provided; Fig. 3 is a cross sectional view depicting a conventional surface light source device which includes a lamp holder; Fig. 4 is a cross-sectional view illustrating still another conventional surface light source device which has a light reflecting member for covering a light source; Figs. 5 and 6 are schematic views depicting a conventional connecting means for connecting a lamp holder to a light conducting member; Fig. 7 is a graph showing a brightness distribution on the light emitting surface of the conventional surface light source device shown in Figs. 5 and 6; Fig. 8 is a cross-sectional view illustrating a surface light source device according to an embodiment of the present invention; Fig. 9 is a cross-sectional view representing a prismatic sheet in an enlarged scale, which is used in the surface light source device illustrated in Fig. 8; Fig. 10 is a graph depicting an example of the brightness distribution with respect to an angle of the light emitted from the conventional surface light source device shown in Fig. 1 when the amount of the spherical silicon grains to be mixed in the transparent ink is varied; Fig. 11 is a graph showing an example of the brightness distribution with respect to an angle of the light emitted from the surface light source device shown in Fig. 8 when the amount of the spherical silicon grains to be mixed in the transparent ink is varied and a prismatic sheet corresponding to the mixed amount of the silicon grains is provided. Best Mode for Carrying Out the Invention Fig. 8 is a schematic view showing an embodiment of the surface light source device according to the present invention. In Fig. 8, the numerical reference 1 denotes a linear light source, 2 a light conducting member, 3 a light diffusing plate, 4 a light reflecting member, and 6 represents a pattern having a light diffusing characteristic. The construction of these members is the same as that of the conventional surface light source device, which is shown in Fig. 1. However, in this embodiment, a prismatic sheet 5 is further arranged on the light diffusing plate 3, and a pattern 6 is formed by using transparent ink, in which 20% - 80% in weight of spherical silicon grains having their diameters of 1 - 10 micrometers are mixed. The apical angle α of each of the prismatic portions of the prism sheet, which is shown in Fig. 9, is selected from the range of 80 - 100 degrees. Fig. 10 is a graph showing brightness distributions on the light diffusing plate 3 with respect to an angle  , which is measured in the conventional surface light source device shown in Fig. 1. It should be noted that the same patterns are formed on the light conducting members by printing, etc. varying the amount of the spherical silicon grains to be mixed in the transparent ink for forming the pattern. In this graph, the curves a, b, c and d show the measurement results of the brightness distribution with respect to the light incident angle  , under the condition that the patterns are formed on the light conducting members, by varying the mixing ratio of the spherical silicone grains to be mixed of 20%, 40%, 60% and 80% in weight, respectively. It should be noted that the angle  is determined in such a manner that the normal to the light emitting surface of the light conducting member 2 is taken at zero degree and the inclination from the perpendicular toward the light source 1 side is taken as minus and the inclination toward the opposite side of the light source 1 is taken as plus. As apparent from Fig. 10, in accordance with the amount of the spherical silicon grains mixed into the transparent ink, the brightness distribution of the emitting light with respect to the angle  is varied. Therefore, by studying the brightness distribution of the emitting light with respect to the angle  and then selecting a suitable prism sheet which corresponds to the brightness distribution, it becomes possible to realize a surface light source device in which a great amount of light is emitted from the light conducting member in the perpendicular direction with respect to the light incident surface of the member. Fig. 11 shows the experimental results of the brightness distribution of the emitting light with respect to the angle  in the surface light source device where a suitable prismatic sheet is used. The type of prism sheet is selected taking the variation of the brightness distribution with respect to the angle  of the emitting light due to the difference of the amount of spherical silicon grains to be mixed in the transparent ink into consideration. In Fig. 11, the curve e represents the brightness distribution with respect to the angle  of the emitting light. The brightness distribution is measured under the condition that a prismatic sheet made of poly carbonate having a diffractive index of 1.585 is disposed on the light diffusing plate 3. It should be noted that 20% in weight of the spherical silicon grains are mixed into the transparent ink and the apical angle of the prismatic portion of the prismatic sheet is 80 degrees. The curve f represents a brightness distribution of the emitting light with respect to the angle  . The brightness distribution is measured under the condition that a prismatic sheet made of poly carbonate having a diffractive index of 1.585 is disposed on the light diffusing plate 3, but 80% in weight percents of the spherical silicon grains are mixed into the transparent ink and the apical angle of the prismatic portion is 90 degrees; the curve g represents a brightness distribution measured under the condition that a prismatic sheet made of poly carbonate having a diffractive index of 1.585 is disposed on the light diffusing plate 3, but 80% in weight of the spherical silicon grains are mixed in the transparent ink and the apical angle of the prismatic portion is 100 degrees; and the curve h represents a brightness distribution measured under the condition that the prismatic sheet made of poly ethylene terephthalate having a diffractive index of 1.575 is disposed on the light diffusing plate 3, but 80% in weight of the spherical silicon grains are mixed into the transparent ink and the apical angle of the prismatic portion is 95 degrees. In all of the above mentioned four cases, the brightness becomes greatest when the angle  of the emitting light is 0 degree. It should be noted that the combinations of the mixed amount of silicon grains and the apical angle of the prismatic sheet correspond to the curves e , f , g and h , respectively. In addition to this, the values of the brightness are greater than those shown in Fig. 10. Particularly, in the case of curve e) wherein 20% in weight of the silicon grains are mixed into the transparent ink and the prism sheet made of poly carbonate having a diffractive index of 1.585, and having an apical angle of 80 degrees is arranged on the light diffusing plate 3, a remarkable effect can be obtained. As stated in above, in the surface light source device according to the present invention, the brightness distribution becomes greatest in a direction perpendicular to the light emitting surface of the light conducting member. Therefore, when the device is used in the back light system of the liquid crystal display device, it is possible to observe the liquid crystal display with an extremely bright image surface. In the present invention explained in the above, the silicon grains having their diameters of 1 to 10 micrometers are mixed into the transparent ink for forming the pattern. If the diameter of the grain is smaller than 1 micrometer, a color separation would be caused and then the emitting light would be colored. Such a device could not be used for the back light system of the liquid crystal display devices. Contrary to this, if the diameter of the grain is greater than 10 micrometers, it would cause a problem that when an operator was to observe the light emitting surface of the surface light source device with naked eyes, the operator could recognize that the brightness on the part where the silicon grains exist is different from that on the other part. It is not only inconvenient for using the device as a brightening device but also causes the other problem that it becomes difficult to print the pattern by using the ink including such large silicon grains and thus the printing accuracy decreases. The most suitable amount of silicon grains to be mixed into the transparent ink is between 20% to 80% in weight. If the amount is less than 20% in weight or greater than 80 in weight, the effect by the silicon grains mixed into the transparent ink becomes unremarkable. Further, the apical angle of each prismatic portion of the prism sheet should be selected from the range of 80 to 100 degrees. If the apical angle is less than 80 degrees, an amount of the light exceeding over the critical angle increases, because of poly carbonate. The ability of the prism sheet is determined by the refracting index of the material of the prism sheet and the apical angle of each prismatic portion of the prism sheet. Therefore, in order to respond to the variation of the distribution of the angle of the emitting light, the type of the prismatic sheet should be selected in a suitable manner in accordance with the amount of silicon grains mixed into the transparent ink. The present inventor carried out the invention by firstly studying the relation among the diameter and the amount of the silicon grains to be mixed into the transparent ink, which is used for printing a pattern on the reflecting member side surface of the light conducting member of the surface light source device, and an amount of light emitted from the light emitting surface of the light conducting member, and its distribution (under the condition that the prism sheet is not provided); and further studying the suitable combination of the diameter and the amount of silicon grains to be mixed into the transparent ink and the prism sheet having different apical angle on the basis of the result of the first study. The purpose of the present invention is achieved in that the pattern is formed on the light conducting member by using the transparent ink in which the spherical silicon grains, whose diameters and amount are selected from the above mentioned ranges, are mixed, and the prism sheet having a multiplicity of prismatic portions whose apical angle is selected from the above mentioned range is arranged on the light conducting member.

Claims: A surface light source device comprising a light conducting plate (2) made of a transparent material, a line an light source (1) disposed in front of a lateral surface (2a) of said light conducting plate a light diffusing member (3) disposed in front of a major surface (26) of said light conducting plate intended to extract light travelling there inside, a light reflecting member (4) disposed in front of the other major surface (2c) opposite to said light emitting major surface (2b), and a pattern (6) having light diffusion characteristics formed on said opposite major surface (2c) of said light conducting plate, wherein said pattern (6) is formed of a half-transparent ink in which fine spherical silicon grains are mixed, characterised in that said fine spherical silicon grains have each a particle diameter of 1 to 10 micrometers, and in that a prismatic sheet (5) having a multiplicity of fine prismatic surfaces is further provided in front of said light diffusing member (3). A surface light source device according to Claim 1, characterised in that said spherical silicon grains are mixed in the transparent ink in an amount of 20% to 80% in weight; and an apical angle of each prismatic surface of said prismatic sheet is in a range of 80 to 100 degrees. A surface light source device according to Claim 1 or 2, characterised in that said spherical silicon grains are mixed in the transparent ink in an amount of 20% in weight. A surface light source device according to one of Claims 1, 2 or 3, characterised in that said prismatic sheet is made of poly carbonate having a refractive index of 1.585.

Inventors: ARAI TAKAYUKI, ISHIKAWA TSUYOSHI, OHTA TETSUFUMI, YOKOYAMA KAZUAKI, ARAI, TAKAYUKI, ISHIKAWA, TSUYOSHI, OHTA, TETSUFUMI, YOKOYAMA, KAZUAKI
Applicants: ENPLAS CORP, ENPLAS CORPORATION
Application year: 1994
Main classification: G02B6
IPC codes: F21V8, G02B6